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Organic Chemistry
Chemistry Regents June 2010 - Question 21 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: (1)

Why? Organic chemistry (or carbon chemistry) by definition is the study of carbon-based compounds, carbon and hydrogen compounds (hydrocarbons) and other derivatives of carbon and hydrocarbons. Organic compounds may contain other elements such as chlorine, nitrogen and oxygen, but these only help to form functional groups associated with carbon compounds.

Answering the Question:

To answer the question, it is important to know what defines an organic compound. Organic compounds are defined by the presence of carbon in there chemical structure. This class of compound is very well known, as many of them are present in living systems on earth. This leaves (1) as the only possible answer.
 
Chemistry Regents June 2010 - Question 23 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: butyne

Why? Organic chemistry is an extremely diverse field of chemistry focusing on carbon chemistry or carbon related molecules and there reactions. The types of elements bonded to a carbon atom and the type of bonds shared, will determine the properties of the molecule and hence the category that molecule will assigned.

A carbon ≡ carbon triple bond is representative of a group of organic compounds called alkynes. All hydrocarbon members of this functional group have the suffix -yne in their name. The prefix of the name is dependent on the total number of carbon atoms in the molecule and the type and position in the main carbon chain, and any side groups of hydrocarbons. For example, if the carbon compound has a carbon-to-carbon triple bond and is composed of only two (2) carbon atoms, it will have a prefix of eth denoting two (2) carbon atoms and a suffix of -yne, denoting the functional group of the molecule. The final name of the compound would be ethyne.

There are three (3) main groups of hydrocarbons, based on the number of carbon-to-carbon bonds present in the molecule. If the hydrocarbons consist of only carbon - carbon single bonds, then they are categorized as alkanes. However, if there is a carbon = carbon double bond present in the hydrocarbon, it is categorized as an alkene.

Answering the Question:

To determine the answer to this question, an understanding of carbon chemistry (organic chemistry) is necessary. In particular it is important to have knowledge of the functional groups present in organic chemistry, then the answers can be analyzed in turn.

(1)   Butane is a four-carbon molecule containing only carbon-to-carbon single bonds. The fact that butane only has carbon-to-carbon single bonds can be determined by analyzing the suffix of the compound. Butane is an alkane; this group is in the category of compounds possessing only single bonds.

(2)   Butanone belongs to the chemical group known as ketones. This category of organic molecules has a carbon covalently double bonded to oxygen and also two hydrocarbon groups.

(3)   Butene is another four-carbon compound, however it differs from butane by having a carbon = carbon double bond.

Based on the functional groups present on the molecules, Answer (4), butyne, is the only possible answer.
 
Chemistry Regents June 2010 - Question 36 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: (2)

Why? An empirical formula is a chemical formula that contains the lowest whole number of atoms for a class of compound. For example C2H4O2 can also be written CH3COOH, which shows the functional group as COOH or organic acid, the simplest of which is acetic acid. One possible method of working out the correct molecule is to create a diagram of the formula. Since carbon will share four of its electrons, it will only form four (4) single bonds. This is important when drawing an organic molecule to determine if the correct number of bonds is being formed.

Answering the Question:

To answer the question, it helps to determine if the whether the molecule does contain the lowest number of atoms.

Answer (1) C6H12O6 - is the general formula for most hexose sugars. A popular member of this group is the sugar glucose. Hexose sugars can be written:

CH=O-CHOH-CHOH-CHOH-CHOH-CH2OH

While this molecule does meet the requirement of being a molecular formula, it is not an empirical formula as it is not the lowest whole number of carbon atoms.

Answer (2) C2H4O2 - is both the empirical formula and molecular formula for ethanoic acid (acetic acid) and glycoaldehyde.

CH3-C=OOH - Ethanoic acid

OHCH2-CH=O - Glycoaldehyde

Both molecules contain the smallest whole number of atoms for their class,

Answer (3) C3H8O - This molecule is the molecular formula for the alcohol propanol. This molecule belongs to the group known as alcohols, with the functional group R-OH. However propanol is not the smallest of the alcohols. Methanol has the general formula CH3OH, which is also the empirical formula of alcohols.

CH3-CH2-CH2OH - Propanol

Answer (4) C4H8 - This molecule follows the molecular formula for alkenes, since it has four carbon atoms, then it would be most likely butene.

CH3-CH=CH-CH3 - 2-Butene

Butene C4H8 is a molecular formula, but again is not an empirical formula as member of the alkenes with the lowest count of atoms is ethene (C2H4). Therefore, answer (2) is the only answer that meets both the requirements of being a molecular formula and an empirical formula. If you did not know the possible classes of compounds a good guess would have been to choose the answer with the lowest number of carbon atoms and determine of all the carbon bonds were complete.
Last Updated on Tuesday, 07 June 2011 14:24
 
Chemistry Regents June 2010 - Question 38 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: (3)

Why? A non-polar bond is formed when two identical atoms form a bond. Molecules such as Cl2, F2, Br2 and I2 are all held together by non-polar covalent (sharing of electrons) bonds. A polar bond is formed when two non-identical atoms form a covalent bond or when an asymmetrical molecule is formed as a result of the bond. Bonds such as the CH and CO are covalent and polar. The reason for the polarity is mainly do to the difference in how strongly one atom will attract electrons versus how the other atom will. If two atoms forming a covalent bond are identical, they will exert an identical force of attraction on the electrons forming the bond. While different atoms will pull differently creating a dipole or a slightly more negative charge around the atom more strongly attracting the electrons and a slightly less negative for those not as strongly attracting the electrons.

Therefore, when looking for a non-polar covalent bond, it is important to look for a bond between two identical atoms. Answer (3) is the only molecule with two identical atoms bonding in the form of a C-C bond. If more than one molecule had a C-C bond, then it would be just as important to look closely at the symmetry of the entire molecule. Symmetrical molecules will not form polar bonds because there is no net pull of electrons on either side of the atoms forming the covalent bond.

Answering the Question:

To answer this question, it is very important to pay attention to what exactly the question is asking. The question asks about the molecule, which contains a non-polar bond, and not which molecule is non-polar. To understand the difference, it is important to understand what makes a bond polar. Answer (1) contains a C-N bond: this bond is between two very different molecules and would cause a dipole to develop as a result of the bond between these two atoms. The difference between the C and N means that one is not a possible answer. Answer (2) is also not correct as its bonds are between two very different atoms. Although the symmetry of the molecule would reduce polarity, the bond between the C and H is a polar one. Both atoms are different and would attract electrons to a different extent. Answer (3) contains C-H bonds, which are polar bonds, however, it also contains a C-C bond, with each of the bonding carbons containing the same number and type of bonds as the other. This means that the C-C bond would not attract the electrons, to a greater degree, than the other. Therefore, the C-C bond present in answer (3) is a non-polar covalent bond. Answer (4) contains both C-H bonds and C-O bonds. Therefore, the difference in atoms means that polar bonds will be formed since the molecule will have no symmetry and identical atoms bonding to counter the polarity. This leaves answer (3) as the correct answer.
Last Updated on Tuesday, 07 June 2011 14:30
 
Chemistry Regents June 2010 - Question 44 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: (3)

Why? An unsaturated hydrocarbon is an organic molecule with at least one carbon to carbon double or triple bond and composed entirely of only carbon and hydrogen atoms (hydrocarbon). Those hydrocarbons, which contain a carbon-to-carbon double bond (C = C), are called alkenes, while those with carbon-to-carbon triple bonds (C ≡ C) are call alkynes.

Answering the Question:

Answer (1) belongs to a group of compounds called aldehydes and contains a carbon to oxygen double bond. This results in answer (1) being incorrect for two reasons; 1) it is not a hydrocarbon, as it contains an oxygen molecule and 2) it would not be considered unsaturated since the double bond is not present between two carbon atoms.

Answer (2) belongs to the group of compounds called alkanes. This group of hydrocarbons contains no carbon-to-carbon double or triple bonds and hence would be considered a saturated hydrocarbon.

Answer (3) is the only hydrocarbon with a carbon-to-carbon double or triple bond. It belongs to the group of compounds known as alkenes, which are characterized by the presence of a carbon-to-carbon double bond. Alkenes meet both the requirements of being a hydrocarbon and being unsaturated.

Answer (4) is typical of the group called ether. They are identified by the carbon-to-oxygen-to-carbon bond within the molecule. This molecule, therefore, does not meet the requirements of the question. It cannot be classified as a hydrocarbon, as it contains an oxygen atom, and it does not contain any carbon-to-carbon double or triple bonds.

Therefore, the best possible answer is answer (3).
 
Chemistry Regents June 2010 - Question 57 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: R-NH2 or R-COOH

Why? Glycine is an organic compound found in proteins and is in fact one of the building blocks of some proteins. Glycine is an amino acid, this means it carries two functional groups; 1) an amine group R-NH2 and 2) a carboxylic acid group R-COOH, where R is a hydrocarbon group (eg CH3CH2-). Amino acids have the general structure H2N-CH-R-COOH and are essential for life as they are the building blocks for proteins. Amino acids at a certain pH, called the isoelectric point, forms a zwitterion, which has both a positive and negative charge.

+H3N-CH-R-COO-

Zwitterions have the ability to react as both an acid and a base.

Answering the Question: To answer the question it is necessary to know what group glycine belongs to. However if this is not known, writing out the given general formula will also help. The main carbon chain is composed of carbon and hydrogen only, all other groups attached to the carbon and hydrogen chain can be considered functional groups.

Therefore, NH2 and COOH are the only possible options in the case of glycine.
 
Chemistry Regents June 2010 - Question 58 PDF Print E-mail
Written by The Chemistry Wizard   
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image058-answer
 
Chemistry Regents June 2010 - Question 72 PDF Print E-mail
Written by The Chemistry Wizard   

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Answer: Sodium Carbonate (Na2CO3)

Why? IUPAC is the International Union of Pure and Applied Chemistry. This union is currently responsible for the rules governing the naming of inorganic and organic compounds. As a result there is currently a set of rules governing the naming of inorganic compounds:

  1. Single atom negatively charged ions (anions) are named with the suffix ide, as in hydride. fluoride, chloride and bromide.
  2. Compounds with a positive ion (cation) are named according to the name of the cation, followed by the name of the anion. For example NaCl is sodium chloride and CaF2 is calcium fluoride.
  3. Cations capable of having more than one oxidation state are usually given a Roman numeral designation according to their current oxidation state. For example, Cu2SO4 (Which has the Cu+ ion) would be written copper (I) Sulfate or Cu(I)SO4. While CuSO4 which contains a Cu2+ ion, would be written copper (II) sulfate or Cu(II)SO4.
  4. Oxyanions (anions such as NO2-, NO3-, CO32-) are named with ite or ate depending on the quantity of oxygen. If two oxygen atoms are present, then it is labeled ite. Therefore, NO2-, would be labeled nitrite, while those species with three oxygen atoms would be labeled with the suffix ate. Therefore, NO3- would be called nitrate, as it has three oxygen atoms making up the anion.

Answering the Question:

The IUPAC name helps to keep consistency in the naming convention (nomenclature) of chemical species. This results in a standard naming convention that describes the structure of the structure of the name compound. Therefore, the molecule Na2CO3 includes sodium, which would start with the prefix being designated to the name of the cation, in this case sodium. It also dictates that the anion (CO32-) would be referred to as carbonate, as it has three oxygen atoms attached. Therefore, the total IUPAC would read sodium carbonate.
Last Updated on Wednesday, 08 June 2011 09:34
 


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